Looks like a countersink-counterbore. I don't know if it's factory or not, but my guess is that the counterbore is just to clean up the edge (from the casting) and the countersink is so the lifter just drops in. I can't imagine there are more than a couple thousandths of an inch (.002"?) clearance for the lifter, so it wouldnt want to drop into a normal counterbore.